Increase Watchdog and PWM Unittest coverage

[rajkan01]

Description of defect

Watchdog unit test is only covered 76% of Watchdog.cpp and extend this unittest to cover the missed the line number 47,50,61,73,75-77,80,82 refer
index.drivers_source_Watchdog.cpp.html.pdf

PwmOut unit test is only covered 70% of PwmOut.cpp and extent the unit test to cover the missed the line number 39,41-43,66,68-70,73,75-77,95,97-99,102,104-106,109,111-113,116,118-121 refer index.drivers_source_PwmOut.cpp.html.pdf

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Target(s) affected by this defect ?

N/A

Toolchain(s) (name and version) displaying this defect ?

N/A

What version of Mbed-os are you using (tag or sha) ?

Latest

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

N/A

How is this defect reproduced ?

View test coverage report

[ciarmcom]

@rajkan01 thank you for raising this issue.Please take a look at the following comments:

What target(s) are you using?
What toolchain(s) are you using?
It would help if you could also specify the versions of any tools you are using?
How can we reproduce your issue?

NOTE: If there are fields which are not applicable then please just add 'n/a' or 'None'. This indicates to us that at least all the fields have been considered.
Please update the issue header with the missing information, the issue will not be mirrored to our internal defect tracking system or investigated until this has been fully resolved.

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-3953